Apparatuses, systems, and methods for probabilistic transmission of device-to-device (d2d) discovery messages

ABSTRACT

Embodiments described herein relate generally to techniques for device discovery for device-to-device (D2D) communications. A user equipment (UE) may receive a transmission probability (e.g., from an evolved Node B (eNB)) for transmission of a discovery medium access control (MAC) protocol data unit (PDU) for D2D communications. The UE may determine a pseudo-random number based on an identifier of the UE, information in the discovery MAC PDU, or information associated with a discovery period. The UE may compare the pseudo-random number with the transmission probability to determine whether to transmit the discovery MAC PDU in the discovery period. Another UE may also determine the pseudo-random number to determine whether the UE is to transmit the discovery MAC PDU in the discovery period. Other embodiments may be described and claimed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 62/055,600, entitled “METHODS FORPROBABILISTIC TRANSMISSION FOR D2D DISCOVERY” and filed Sep. 25, 2014,and to U.S. Provisional Patent Application No. 62/069,711, entitled“METHODS FOR PROBABILISTIC TRANSMISSION FOR D2D DISCOVERY” and filedOct. 28, 2014, the entire disclosures of which are incorporated hereinby reference.

FIELD

Embodiments of the present invention relate generally to the technicalfield of data processing, and more particularly, to techniques fordevice-to-device communication.

BACKGROUND

A user equipment (UE) may use device-to-device (D2D) communications tocommunicate directly with another UE, e.g., without routing thecommunications through an evolved Node B (eNB). In Type 1 D2D discoveryprocedure, the network may allocate a pool of discovery resources in theform of periodically occurring sets of physical time-frequency resourcesfor transmission of D2D discovery medium access control (MAC) packetdata units (PDUs). A transmitting UE randomly selects an individualresource from within the discovery pool for each discovery period fortransmission of a D2D discovery MAC PDU. When there are a large numberof UEs participating in the D2D discovery procedure in the pool ofdiscovery resources, there may be a high probability of collisions,thereby leading to a significant decrease in the overall performance interms of the number of devices discovered by D2D UEs.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention are illustrated by way of example andnot by way of limitation in the figures of the accompanying drawings inwhich like references indicate similar elements. It should be noted thatreferences to “an” or “one” embodiment of the invention in thisdisclosure are not necessarily to the same embodiment, and they may meanat least one.

FIG. 1 is a block diagram showing a wireless communication environmentincluding user equipments (UEs) and an evolved Node B (eNB), inaccordance with various embodiments.

FIG. 2 is a flow diagram illustrating a method for D2D discovery, inaccordance with various embodiments.

FIG. 3 is a flow diagram illustrating another method for D2D discovery,in accordance with various embodiments.

FIG. 4 is a block diagram illustrating a computing device adapted tooperate in a wireless communication network, in accordance with variousembodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof wherein like numeralsdesignate like parts throughout, and in which is shown by way ofillustration embodiments that may be practiced. It is to be understoodthat other embodiments may be utilized and structural or logical changesmay be made without departing from the scope of the present disclosure.Therefore, the following detailed description is not to be taken in alimiting sense, and the scope of embodiments is defined by the appendedclaims and their equivalents.

Various operations may be described as multiple discrete actions oroperations in turn, in a manner that is most helpful in understandingthe claimed subject matter. However, the order of description should notbe construed as to imply that these operations are necessarily orderdependent. In particular, these operations may not be performed in theorder of presentation. Operations described may be performed in adifferent order than the described embodiment. Various additionaloperations may be performed and/or described operations may be omittedin additional embodiments.

For the purposes of the present disclosure, the phrases “A or B” and “Aand/or B” means (A), (B), or (A and B). For the purposes of the presentdisclosure, the phrase “A, B, and/or C” means (A), (B), (C), (A and B),(A and C), (B and C), or (A, B, and C).

The description may use the phrases “in an embodiment,” or “inembodiments,” which may each refer to one or more of the same ordifferent embodiments. Furthermore, the terms “comprising,” “including,”“having,” and the like, as used with respect to embodiments of thepresent disclosure, are synonymous.

As used herein, the term “circuitry” may refer to, be part of, orinclude an Application Specific Integrated Circuit (ASIC), an electroniccircuit, a processor (shared, dedicated, or group), and/or memory(shared, dedicated, or group) that execute one or more software orfirmware programs, a combinational logic circuit, and/or other suitablehardware components that provide the described functionality. In someembodiments, circuitry may be implemented in, or functions associatedwith the circuitry may be implemented by, one or more software orfirmware modules.

FIG. 1 schematically illustrates a wireless communication environment100 in accordance with various embodiments. The environment 100 mayinclude a UE 104, a UE 108, and an evolved Node B (eNB) 112. Inembodiments, the UE 104 and UE 108 may communicate wirelessly with oneanother via device-to-device (D2D) communications. In D2Dcommunications, the UE 104 and UE 108 may wirelessly communicate (e.g.,transmit or receive) signals directly between one another (e.g., withoutrouting the signals through the eNB 112).

In some embodiments, the UE 104 and/or UE 108 may also be in wirelesscommunication with an access node such as the evolved node B (eNB) 112.The eNB 112 may be part of a 3rd Generation Partnership Project (3GPP)long-term evolution (LTE) network (or an LTE-Advanced (LTE-A) network).In particular, the eNB 112 may be part of a radio access network (RAN)of the LTE/LTE-A network, such as an evolved universal terrestrial radioaccess network (E-UTRAN). The E-UTRAN may be coupled with a core networksuch as an Evolved Packet Core (EPC) that performs various managementand control functions of the LTE/LTE-A network and further provides acommunication interface between various RANs and other networks.

UE 104 may include communication circuitry 116, control circuitry 120,radio transceiver 122, and one or more antennas 124. Communicationcircuitry 116 may interface with the radio transceiver 122 to receiveD2D signals from and/or send D2D signals to the UE 108 via the one ormore antennas 124. The communication circuitry 116 may also interfacewith the transceiver 122 to receive downlink transmissions from eNB 112and transmit uplink transmissions to eNB 112 via the one or moreantennas 124. In some embodiments, the communication circuitry 116 maybe baseband circuitry, and the transceiver 122 may be radio frequency(RF) circuitry. The control circuitry 120 may be coupled tocommunication circuitry 116, and may be configured to decode and encodeinformation transmitted in signals communicated between the UE 104 andthe UE 108 and/or eNB 112. Control circuitry 120 may further beconfigured to perform any portion of the processes described below.

UE 108 may include similar components and/or functionality to UE 104.For example, UE 108 may include communication circuitry 128, controlcircuitry 132, radio transceiver 134, and one or more antennas 136.Communication circuitry 128 may interface with the radio transceiver 134to receive D2D signals from and/or send D2D signals to the UE 104 viathe one or more antennas 136. The communication circuitry 128 may alsointerface with the radio transceiver 134 to receive downlinktransmissions from eNB 112 and transmit uplink transmissions to eNB 112via the one or more antennas 136. Control circuitry 132 may be coupledto communication circuitry 128, and may be configured to decode andencode information transmitted in signals communicated between the UE108 and the UE 104 and/or eNB 112. Control circuitry 132 may further beconfigured to perform any portion of the processes described below.

The UE 104 and/or UE 108 may be any type of computing device equippedwith broadband circuitry and adapted to communicate with another UE viaD2D communications and/or to operate on a cell of a wirelesscommunication network according to, for example, one or more 3GPPtechnical specifications. The wireless cell may be provided by the eNB104.

The eNB 112 may include communication circuitry 140 to interface withradio transceiver 144 to receive uplink transmissions from UE 104 or UE108 via one or more antennas 148 and transmit downlink transmissions toUE 104 or UE 108 via the one or more antennas 148. eNB 112 may alsoinclude control circuitry 152 coupled with communication circuitry 140.In embodiments control circuitry 152 may be configured to decode andencode information transmitted in signals communicated between the UE104 or UE 108 and eNB 112.

In various embodiments, the eNB 112 (e.g., the control circuitry 152 ofthe eNB 112) may allocate a discovery pool of discovery resources thatare to be used by UEs associated with a cell provided by the eNB 112(e.g., UEs for which the eNB 112 provides the serving cell or campingcell), such as the UE 104 and/or UE 108, for D2D discovery (e.g., Type 1D2D discovery). The discovery resources of the discovery pool maycorrespond to physical resources in the time-frequency domain. Thediscovery pool may include discovery periods that include sets ofphysical resources. The discovery periods may occur periodically.

In some embodiments, the discovery resources may correspond to resourcesof the uplink (UL) spectrum that would otherwise be allocated for ULtransmissions from UEs to the eNB 112. For example, the discoveryresources may occur periodically within the uplink (UL) spectrum (e.g.,in frequency-division duplexing (FDD) deployments) or on a subset ofuplink subframes (e.g., in time-division duplexing (TDD) deployments).In other embodiments, the discovery resources may include otherresources of the licensed spectrum or may include resources of otherwireless spectrum, such as unlicensed spectrum, that is not used forcommunications between the eNB 112 and UEs of the cell (e.g., the UEs104 and 108). In some embodiments, the eNB 112 may signal the allocateddiscovery pool to the UE 104 and UE 108 (e.g., using common RadioResource Control (RRC) signaling, such as in System Information Block(SIB) type 19).

In various embodiments, the UEs (e.g., the UE 104 and/or UE 108) thatare interested in discovering other D2D UEs with which to communicatemay transmit a discovery medium access control (MAC) packet data unit(PDU) using one or more discovery resources in a discovery period of thediscovery pool. The UE may randomly select one or more discoveryresources of the discovery period on which to transmit the discovery MACPDU. The UE may use a plurality of discovery resources, for example, ifrepeated transmission of discovery signals within a discovery period isconfigured.

In various embodiments, the eNB 112 may further signal a transmissionprobability (e.g., as part of the discovery pool configurationinformation transmitted using common RRC signaling, such as SIB type19), to the UEs 104 and/or 108. The transmission probability maycorrespond to a probability with which the UE 104 or UE 108 shouldtransmit its associated discovery MAC PDU. For example, in oneembodiment, the transmission probability may be 0.25, 0.5, 0.75, or 1. Atransmission probability of 1 may indicate that the UE may transmit adiscovery MAC PDU in every discovery period of the allocated discoverypool, while a transmission probability of 0.75 may indicate that the UEwill transmit a discovery MAC PDU in about 3 out of 4 discovery pools.The transmission probability may be used to reduce the number ofdiscovery MAC PDUs that are transmitted in the discovery pool to preventor reduce collisions between the discovery MAC PDUs. The eNB 112 (e.g.,the control circuitry 152) may select the transmission probability basedon any suitable factors, such as the loading of the cell from D2D UEsthat are participating in D2D discovery.

In some embodiments, the transmission probability may be the same forall UEs associated with the cell. In other embodiments, the transmissionprobability may be UE-specific.

Various embodiments will be described with reference to the UE 104transmitting a discovery MAC PDU and the UE 108 receiving the discoveryMAC PDU. In embodiments, the UE 104 may also use the techniquesdescribed with respect to the UE 108, and the UE 108 may also use thetechniques described with respect to the UE 104. For example, the UE 104may also receive a discovery MAC PDU that is transmitted by the UE 108.

In various embodiments the control circuitry 120 of the UE 104 mayobtain, via the communication circuitry 116, an allocation of discoveryresources for a discovery pool to be used for D2D discovery. The controlcircuitry 120 may also obtain, via the communication circuitry 116, atransmission probability for transmission of a discovery MAC PDU. Thetransmission probability may be associated with one or more periods ofthe discovery pool.

In various embodiments, the control circuitry 120 may determine apseudo-random number and compare the pseudo-random number with thetransmission probability to determine whether to send the discovery MACPDU in the discovery period. For example, the control circuitry 120 maydetermine to send the discovery MAC PDU in the discovery period if thepseudo-random number is less than the transmission probability, and mayrefrain from sending the discovery MAC PDU in the discovery period ifthe pseudo-random number is greater than the transmission probability.In some embodiments, the pseudo-random number and the transmissionprobability may both be values from 0 to 1.

In some embodiments, a plurality of discovery periods may be groupedtogether, and the transmission probability may be determined based onthe number of discovery periods that are grouped together. The controlcircuitry 120 of the UE 104 may determine whether to transmit thediscovery MAC PDU within the plurality of discovery periods (e.g., inone of the discovery periods of the plurality of discovery periods)based on a comparison of the pseudo-random number and the transmissionprobability. For example, the transmission probability may be 1/N for agroup of N discovery periods. The UE 104 may transmit once within thegroup of N discovery pools if the generated pseudo-random number is lessthan 1/N. In some embodiments, the eNB 112 may determine the number ofdiscovery pools that are grouped together, and may signal the number ofdiscovery pools to the UE 104 and/or UE 108.

In various embodiments, the control circuitry 120 may determine thepseudo-random number based on one or more parameters, such as anidentifier of the UE 104, information in the discovery MAC PDU, orinformation associated with the discovery pool. The identifier of the UE104 may be, for example, a D2D UE identity that is a function of aninternational mobile subscriber identity (IMSI) of the UE 104, a D2D UEidentity that is a function of a system architecture evolution temporarymobile subscriber identity (S-TMSI) of the UE 104, or a Layer 2identifier (ID) of the UE 104.

The information in the discovery MAC PDU may include a field or bits ofthe discovery MAC PDU. For example, in some embodiments, the controlcircuitry 120 may determine the pseudo-random number based on aproximity services (ProSe) application code and/or a ProSe Function IDof the discovery MAC PDU. The ProSe application code and ProSe FunctionID may be associated with the UE 104. Alternatively, the controlcircuitry 120 determine the pseudo-random number based on another set ofbits of the discovery MAC PDU.

The information associated with the discovery pool may include, forexample, a system frame number (SFN) or subframe number associated withthe discovery pool. For example, in some embodiments, the controlcircuitry 120 may determine the pseudo-random number based on a functionof the system frame number and the subframe number (e.g., index) of thefirst subframe included in the discovery pool.

In some embodiments, the control circuitry 120 may determine thepseudo-random number based on a pseudo-random binary sequence (PRBS).For example, the control circuitry 120 may determine a PRBS based on theidentifier of the UE 104, the information in the discovery MAC PDU, orthe information associated with the discovery pool. In some embodiments,the control circuitry 120 may initialize the PRBS (e.g., determine aninitialization sequence for the PRBS, as further discussed below) basedon the identifier of the UE 104, the information in the discovery MACPDU, or the information associated with the discovery pool.

In some embodiments, the transmission probability and the informationused by the UE 104 to determine the pseudo-random number may also beknown to other UEs, such as the UE 108. Accordingly, the UE 108 mayindependently determine whether the UE 104 is to transmit the discoveryMAC PDU in the discovery period. The determination by the UE 108 mayallow the UE 108 (e.g., the control circuitry 132) to determine whetherthe UE 108 did not receive a discovery MAC PDU from the UE 104 becausethe UE 104 was not supposed to transmit a discovery MAC PDU or becausethe UE 104 has left the cell or is no longer participating in D2Ddiscovery.

For example, the control circuitry 132 may receive, via thecommunication circuitry 128, a first discovery MAC PDU from the UE 104in a first discovery period of a discovery pool. The control circuitry132 may determine whether the UE 104 is to transmit a second discoveryMAC PDU in a second discovery period based on the transmissionprobability associated with the UE 104. The control circuitry 132 maymonitor for the second MAC PDU in the second discovery period based onthe determination.

The control circuitry 132 may determine whether the UE 104 is totransmit the second discovery MAC PDU in the second discovery period,for example, by determining a pseudo-random number and comparing thepseudo-random number with the transmission probability. The controlcircuitry 132 may determine the pseudo-random number based on, forexample, an identifier of the UE 104, information in the first discoveryMAC PDU, or information associated with the second discovery period.

If the UE 108 determines that the UE 104 was supposed to transmit adiscovery MAC PDU in the discovery pool, but the UE 108 did not receivethe discovery MAC PDU, the UE 108 may determine that the UE 104 has leftthe area or is no longer participating in D2D discovery. It is alsopossible that the UE 104 did transmit the discovery MAC PDU, but thatthe UE 108 did not receive it due to resource collisions or half-duplexconflicts. Accordingly, in some embodiments, the UE 108 may determinethat the UE 104 has left the area or is no longer participating in D2Ddiscovery when the UE 108 has not received a discovery MAC PDU for apre-determined number of discovery periods for which the UE 104determines that the UE 108 should have transmitted a discovery MAC PDU.

Additionally, or alternatively, the use of the pseudo-random number asdescribed herein may allow a network entity (e.g., the eNB 112) to testwhether the UE 104 is transmitting the discovery MAC PDU when it issupposed to based on the configured transmission probability.

In some embodiments, the UE 104 and/or UE 108 may adjust thetransmission probability for a discovery pool based on whether the UE104 transmitted a discovery MAC PDU in the preceding discovery pool orin a preceding number of discovery pools. For example, the UE 104 and/orUE 108 may increase the transmission probability for a discovery pool ifthe UE 104 has not transmitted a discovery MAC PDU in the previous twodiscovery pools.

In some embodiments, the discovery MAC PDU transmitted by the UE 104 mayinclude a ProSe application code, a ProSe function ID, and a public landmobile network (PLMN) ID. In some embodiments, the ProSe applicationcode may be 160 bits, the ProSe Function ID may be 8 bits, and/or thePLMN ID may be 24 bits, as shown in Table 1.

TABLE 1 Assessed Field Length Prose 160 bits Application Code ProSe  8bits Function ID PLMN ID  24 bits

Alternatively, the discovery MAC PDU transmitted by the UE 104 mayinclude the fields shown in Table 2. The discovery MAC PDU with contentsaccording to Table 2 may be used, for example, for public safetyapplications.

TABLE 2 Assessed Field Length Purpose Source L2 e.g. 48 To identify asingle UE source of the ID/ProSe UE bits information in the message.This can be ID of source used for subsequent communication or to send areply in Model B of operation Destination e.g. 48 To identify a singleUE or group of L2 ID bits UEs that are intended recipients of theinformation (a single UE in responses for model B). Message  8 bits Typeof discovery message type Prose 64 bits Needed to perform matching tothe Application required service Set/Discovery criterion ID UE mode of 2 bits Defines whether a Public safety ProSe operation UE is acting asa UE-to-network relay, UE-to-UE or both or not acting as relay PLMN ID24 bits The PLMN ID the ProSe UE is attached to. Status bits  4 bitsSome status/maintenance flags.

The fields and number of bits for each field are presented in Tables 1and 2 as examples, and other embodiments of the discovery MAC PDU mayinclude different fields or a different number of bits for a givenfield.

As discussed above, in some embodiments, the UE 104 and/or UE 108 maygenerate a PRBS and determine the pseudo-random number based on thePRBS. In some embodiments, the pseudo-random binary sequence may bedefined by a length-31 Gold sequence. For example, the PRBS generatormay generate an output sequence c(n) of length M_(PN), where n=0, 1, . .. , M_(PN)−1, that is defined by:

c(n)=(x ₁(n+N _(c))+x ₂(n+N _(c)))mod 2

x ₁(n+31)=(x ₁(n+3)+x ₁(n))mod 2

x ₂(n+31)=(x ₂(n+3)+x ₂(n+2)+x ₂(n+1)+x ₂(n))mod 2

where N_(c)=1600, and the first m-sequence is initialized with x₁(0)=1,x₁(n)=0, n=1, 2, . . . , 30. The initialization of the second m-sequenceis denoted by

$c_{init} = {\sum\limits_{i = 0}^{30}\; {{x_{2}(i)} \cdot {2^{i}.}}}$

In some embodiments, the initialization sequence c_(init) may be basedon the identifier of the UE 104, the information in the discovery MACPDU, or the information associated with the discovery pool.

As discussed above, in some embodiments, the pseudo-random number may begenerated based on the SFN and/or a subframe associated with thediscovery pool. For example, using the PRBS c(n), in some embodiments,the UE 104 and/or UE 108 may determine the pseudo-random numberaccording to Equation (1) below:

$\begin{matrix}{p_{UE} = {\sum\limits_{i = 0}^{L - 1}\; {{c\left( {{L \cdot \left( {{10 \cdot {SFN}} + {firstSubFrameIdx}} \right)} + i} \right)} \cdot w^{- {({i + 1})}}}}} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

where p_(UE) is the pseudo-random number, c corresponds to a PRBSgeneration function, SFN is a system frame number associated with thediscovery pool, firstSubFrameIdx is an index of a first subframe of thediscovery pool, and L is a positive integer (e.g., L=10).

In other embodiments, the UE 104 and/or UE 108 may determine thepseudo-random number based on bits of the discovery MAC PDU. Forexample, the pseudo-random number may be determined according toEquation (2):

$\begin{matrix}{p_{UE} = {\sum\limits_{i = 0}^{L - 1}\; {c{\quad{\left( {{L \cdot \left( {\left( {\sum\limits_{j = 0}^{M - 1}\; {{{DiscoveryMsg}(j)} \cdot 2^{i}}} \right){mod}\mspace{14mu} K} \right)} + i} \right)*2^{- {({i + 1})}}}}}}} & {{Equation}\mspace{14mu} (2)}\end{matrix}$

where p_(UE) is the pseudo-random number, c corresponds to a PRBSgeneration function, K is a pre-defined constant, DiscoveryMsg(j), j=0,1, . . . , M−1 corresponds to a length-M set of indices of a codedversion of the discovery MAC PDU or bits of an uncoded version of thediscovery MAC PDU, and L is a positive integer. For example, in someembodiments, DiscoveryMsg(j), j=0, 1, . . . , M−1 may correspond to theM least significant bits (LSBs), M most significant bits (MSBs), oranother set of M bits of the discovery MAC PDU.

In other embodiments, the UE 104 and/or UE 108 may determine thepseudo-random number based on an identifier associated with the UE 104.For example, the pseudo-random number may be determined according toEquation (3):

$\begin{matrix}{p_{UE} = {\sum\limits_{i = 0}^{L - 1}{{c\left( {{L \cdot {UE\_ ID}} + i} \right)}*2^{- {({i + 1})}}}}} & {{Equation}\mspace{14mu} (3)}\end{matrix}$

where UE_ID=IMSI mod K and IMSI is in decimal format, K is a pre-definedconstant (e.g., K=1024), and L is a positive integer (e.g., L=10). Inother embodiments, the UE_ID in Equation (3) may be similarly derivedusing the S-TMSI of the UE 104. The S-TMSI may have a value that rangesfrom 0 to 2⁴¹−1.

In some embodiments, the initialization sequence c_(init) used toinitialize the PRBS may be defined based on the identifier of the UE104, the information in the discovery MAC PDU, or the informationassociated with the discovery pool. For example, in some embodiments,the initialization sequence c_(init) may be defined by Equation (4):

$\begin{matrix}{c_{init} = {{\sum\limits_{i = 0}^{22}{{{ProSeAppCode}(i)} \cdot 2^{i}}} + {\sum\limits_{i = 23}^{30}{{{ProSeFuncID}\left( {i - 23} \right)} \cdot 2^{i}}}}} & {{Equation}\mspace{14mu} (4)}\end{matrix}$

where ProSeAppCode(i), i=0, 1, . . . , 22, corresponds to 23 bits of aProSe Application Code or a ProSe Application ID of the discovery MACPDU, and ProSeFuncID(i−23), i=23, 24, . . . , 30, corresponds to 8 bitsof a ProSe Function ID of the discovery MAC PDU.

In other embodiments, the initialization sequence c_(init) may bedefined by Equation (5):

$\begin{matrix}{c_{init} = {\sum\limits_{i = 0}^{30}{{{DiscoveryMsg}(i)} \cdot 2^{i}}}} & {{Equation}\mspace{14mu} (5)}\end{matrix}$

where DiscoveryMsg(i), i=0, 1, . . . , 30 corresponds to 31 bits of thediscovery MAC PDU (e.g., the 31 LSBs, 31 MSBs, or another set of 31bits).

In other embodiments, the initialization sequence c_(init) may bedefined by Equation (6):

c _(init)=SFN·2⁴+firstSubFrameIdx   Equation (6)

where SFN is a system frame number associated with the discovery periodof the discovery pool, firstSubFrameIdx is an index of a first subframeof the discovery period of the discovery pool. Defining theinitialization sequence as a function of the configuration of thediscovery period helps in reducing the computational complexity atreceiving UEs. A receiving UE may generate a single initializationsequence that can be commonly used towards predicting the transmissionevents in the discovery period from all of the UEs for which thereceiving UE may be monitoring the discovery resources.

In other embodiments, the initialization sequence c_(init) may bedetermined based on Equation (7):

c _(init)=UE_ID   Equation (7)

where UE_ID=IMSI mod K, IMSI is in decimal format, and K is apre-defined constant. The constant K may be not greater than 2³¹. Insome embodiments, the constant K may have a value smaller than 2³¹, suchas 1024. In other embodiments, the UE_ID in Equation (7) may besimilarly derived using the S-TMSI of the UE 104. The S-TMSI may have avalue that ranges from 0 to 241−1.

In some embodiments, the UE 108 may not have knowledge of the identifierof UE 104 that is used by the UE 104 to generate the pseudo-randomnumber. Such embodiments may be used when it is not necessary for the UE108 to know the silencing pattern used by the UE 104 to transmitdiscovery MAC PDUs.

FIG. 2 is flow chart illustrating a method 200 for D2D discovery thatmay be performed by a UE (e.g., UE 104 or UE 108) in accordance withvarious embodiments. In some embodiments, the UE may include one or morenon-transitory computer-readable media having instructions, storedthereon, that when executed cause the UE to perform the method 200.

At 204 of the method 200, the UE may obtain a transmission probabilityfor transmission of a discovery MAC PDU for D2D communications. In someembodiments, the UE may receive the transmission probability from aneNB, such as the eNB 112.

At 208 of the method 200, the UE may determine a pseudo-random numberbased on an identifier of the UE, information in the discovery MAC PDU,or information associated with a discovery period of the allocateddiscovery pool. In some embodiments, the UE may determine a PRBS basedon the identifier of the UE, the information in the MAC PDU, or theinformation associated with the discovery period of the allocateddiscovery pool, and the UE may determine the pseudo-random number basedon the PRBS.

At 212 of the method 200, the UE may compare the pseudo-random numberwith the transmission probability to determine whether to transmit thediscovery MAC PDU in the discovery period. For example, the UE maytransmit the discovery MAC PDU in the discovery period if thepseudo-random number is less than the transmission probability, and maynot send the discovery MAC PDU in the discovery period if thepseudo-random number is greater than the transmission probability.

FIG. 3 is flow chart illustrating a method 300 for D2D discovery thatmay be performed by a UE (e.g., UE 104 or UE 108) in accordance withvarious embodiments. In some embodiments, the UE may include one or morenon-transitory computer-readable media having instructions, storedthereon, that when executed cause the UE to perform the method 300.

At 304 of the method 300, the UE may receive a first discovery MAC PDUfrom another UE (e.g., the UE 108 or UE 104) in a first discovery periodof a discovery resource pool.

At 308 of the method 300, the UE may determine whether the other UE isto transmit a second discovery MAC PDU in a second discovery periodbased on a transmission probability associated with the other UE. Insome embodiments, the UE may receive the transmission probability froman eNB, such as the eNB 112. In some embodiments, the UE may also usethe transmission probability to determine whether to transmit its owndiscovery MAC PDU in the first and/or second discovery period.

At 312 of the method 300, the UE may monitor for the second MAC PDU inthe second discovery period based on the determination.

The UE 104, UE 108, and/or eNB 112 as described herein may beimplemented into a system using any suitable hardware, firmware, and/orsoftware configured as desired. For example, FIG. 4 illustrates, for oneembodiment, an example system 400 that may correspond to a UE (e.g., UE104 or UE 108) or eNB (e.g., eNB 112). Additionally, or alternatively,the system 400 may be adapted to perform one or more of the processesdescribed herein (e.g., method 200 and/or 300). The system 400 mayinclude radio frequency (RF) circuitry 404, baseband circuitry 408,application circuitry 412, memory/storage 416, display 420, camera 424,sensor 428, and input/output (I/O) interface 432, coupled with eachother at least as shown.

The application circuitry 412 may include circuitry such as, but notlimited to, one or more single-core or multi-core processors. Theprocessor(s) may include any combination of general-purpose processorsand dedicated processors (e.g., graphics processors, applicationprocessors, etc.). The processors may be coupled with memory/storage 416and configured to execute instructions stored in the memory/storage 416to enable various applications and/or operating systems running on thesystem 400.

The baseband circuitry 408 may include circuitry such as, but notlimited to, one or more single-core or multi-core processors. Theprocessor(s) may include a baseband processor. The baseband circuitry408 may handle various radio control functions that enable communicationwith one or more radio networks via the RF circuitry 404. The radiocontrol functions may include, but are not limited to, signalmodulation, encoding, decoding, radio frequency shifting, etc. In someembodiments, the baseband circuitry 408 may provide for communicationcompatible with one or more radio technologies. For example, in someembodiments, the baseband circuitry 408 may support communication withan E-UTRAN and/or other wireless metropolitan area networks (WMAN), awireless local area network (WLAN), or a wireless personal area network(WPAN). Embodiments in which the baseband circuitry 408 is configured tosupport radio communications of more than one wireless protocol may bereferred to as multi-mode baseband circuitry.

In various embodiments, baseband circuitry 408 may include circuitry tooperate with signals that are not strictly considered as being in abaseband frequency. For example, in some embodiments, baseband circuitry408 may include circuitry to operate with signals having an intermediatefrequency, which is between a baseband frequency and a radio frequency.

In some embodiments, the communication circuitry 116, communicationcircuitry 128, communication circuitry 140, control circuitry 120,control circuitry 132, and/or control circuitry 152 may be embodied inthe application circuitry 412 and/or the baseband circuitry 408.Alternatively, the communication circuitry 116, 128, and/or 140 may beembodied in the RF circuitry 404.

RF circuitry 404 may enable communication with wireless networks usingmodulated electromagnetic radiation through a non-solid medium. Invarious embodiments, the RF circuitry 404 may include switches, filters,amplifiers, etc., to facilitate the communication with the wirelessnetwork. The radio transceiver 122, radio transceiver 134, and/or radiotransceiver 144 may be embodied in the RF circuitry 404.

In various embodiments, RF circuitry 404 may include circuitry tooperate with signals that are not strictly considered as being in aradio frequency. For example, in some embodiments, RF circuitry 404 mayinclude circuitry to operate with signals having an intermediatefrequency, which is between a baseband frequency and a radio frequency.

In some embodiments, some or all of the constituent components of thebaseband circuitry 408, the application circuitry 412, and/or thememory/storage 416 may be implemented together on a system on a chip(SOC).

Memory/storage 416 may be used to load and store data and/orinstructions, for example instructions 410 which may be configured tocause system 400 to carry out any portion of the processes describedherein (e.g., method 200 and/or 300). Memory/storage 416 for oneembodiment may include any combination of suitable volatile memory(e.g., dynamic random access memory (DRAM)) and/or non-volatile memory(e.g., Flash memory).

In various embodiments, the I/O interface 432 may include one or moreuser interfaces designed to enable user interaction with the system 400and/or peripheral component interfaces designed to enable peripheralcomponent interaction with the system 400. User interfaces may include,but are not limited to, a physical keyboard or keypad, a touchpad, aspeaker, a microphone, etc. Peripheral component interfaces may include,but are not limited to, a non-volatile memory port, a universal serialbus (USB) port, an audio jack, and a power supply interface.

In various embodiments, sensor 428 may include one or more sensingdevices to determine environmental conditions and/or locationinformation related to the system 400. In some embodiments, the sensorsmay include, but are not limited to, a gyro sensor, an accelerometer, aproximity sensor, an ambient light sensor, and a positioning unit. Thepositioning unit may also be part of, or interact with, the basebandcircuitry 408 and/or RF circuitry 404 to communicate with components ofa positioning network, e.g., a global positioning system (GPS)satellite.

In various embodiments, the display 420 may include a display (e.g., aliquid crystal display, a touch screen display, etc.).

In some embodiments, the system 400 may be a mobile computing devicesuch as, but not limited to, a laptop computing device, a tabletcomputing device, a netbook, an ultrabook, a smartphone, etc.Alternatively, or additionally, the system 400 may be a relativelynon-mobile computing device such as, but not limited to, a desktopcomputing device, a set-top box, a gaming console, a media player, asmart metering device, an appliance, a security system (e.g., asurveillance device), and the like. In various embodiments, system 400may have more or fewer components, and/or different architectures.

Some non-limiting Examples are provided below.

Example 1 is an apparatus to be employed by a user equipment (UE), theapparatus comprising: communication circuitry to interface with a radiotransceiver to wirelessly communicate with an evolved Node B (eNB); andcontrol circuitry coupled to the communication circuitry. The controlcircuitry is to: obtain, via the communication circuitry, a transmissionprobability from the eNB for transmission of a discovery medium accesscontrol (MAC) protocol data unit (PDU) for device-to-device (D2D)communications; determine a pseudo-random number based on an identifierof the UE, information in the discovery MAC PDU, or informationassociated with a discovery period; and compare the pseudo-random numberwith the transmission probability to determine whether to send thediscovery MAC PDU in the discovery period.

Example 2 is the apparatus of Example 1, wherein the control circuitryis to: determine a pseudo-random binary sequence (PRBS) based on theidentifier of the UE, the information in the MAC PDU, or the informationassociated with the discovery period; and determine the pseudo-randomnumber based on the PRBS.

Example 3 is the apparatus of Example 2, wherein the control circuitryis to initialize the PRBS based on the identifier of the UE, a field orbits of the discovery MAC PDU, or a system frame number (SFN) or asubframe number associated with the discovery period.

Example 4 is the apparatus of Example 1, wherein the control circuitryis to determine the pseudo-random number based on the identifier of theUE, wherein the identifier is a D2D UE identity that is a function of aninternational mobile subscriber identity (IMSI) of the UE, a D2D UEidentity that is a function of a system architecture evolution temporarymobile subscriber identity (S-TMSI) of the UE, or a Layer 2 identifier(ID) of the UE.

Example 5 is the apparatus of Example 1, wherein the control circuitryis to determine the pseudo-random number based on a field or bits of thediscovery MAC PDU.

Example 6 is the apparatus of Example 5, wherein the control circuitryis to determine the pseudo-random number based on a ProSe applicationcode and a ProSe Function ID of the discovery MAC PDU.

Example 7 is the apparatus of Example 1, wherein the control circuitryis to determine the pseudo-random number based on a system frame number(SFN) or a subframe number associated with the discovery period.

Example 8 is the apparatus of any one of Examples 1 to 7, wherein thediscovery period is a first discovery period and the discovery MAC PDUis a first discovery MAC PDU, and wherein the control circuitry isfurther to adjust the transmission probability for determination ofwhether to transmit a second discovery MAC PDU in a second discoveryperiod based on whether the first discovery MAC PDU was transmitted inthe first discovery period.

Example 9 is the apparatus of any one of Examples 1 to 7, wherein thediscovery period is a first discovery period, and wherein the controlcircuitry is to compare the pseudo-random number with the transmissionprobability to determine whether to transmit the discovery MAC PDU in aset of discovery periods that includes the first discovery period.

Example 10 is an apparatus to be employed by a first user equipment(UE), the apparatus comprising: communication circuitry to interfacewith a radio transceiver to wirelessly communicate with a second UE viadevice-to-device (D2D) communication; and control circuitry coupled tothe communication circuitry. The control circuitry is to: receive, viathe communication circuitry, a first discovery medium access control(MAC) protocol data unit (PDU) from the second UE in a first discoveryperiod; determine whether the second UE is to transmit a seconddiscovery MAC PDU in a second discovery period based on a transmissionprobability associated with the second UE; and monitor for the secondMAC PDU in the second discovery period based on the determination.

Example 11 is the apparatus of Example 10, wherein the control circuitryis to: determine a pseudo-random binary number based on an identifier ofthe second UE, information in the first discovery MAC PDU, orinformation associated with the second discovery period; and compare thepseudo-random number with the transmission probability to determinewhether the second UE is to transmit the second discovery MAC PDU in thesecond discovery period.

Example 12 is the apparatus of Example 11, wherein the control circuitryis to determine the pseudo-random number based on a field or bits of thediscovery MAC PDU.

Example 13 is the apparatus of Example 12, wherein the control circuitryis to determine the pseudo-random number based on a ProSe applicationcode and a ProSe Function ID of the discovery MAC PDU.

Example 14 is the apparatus of Example 11, wherein the control circuitryis to determine the pseudo-random number based on a system frame number(SFN) or a subframe number associated with the discovery period.

Example 15 is the apparatus of any one of Examples 11 to 14, wherein thecontrol circuitry is to: determine a pseudo-random binary sequence(PRBS) based on the identifier of the second UE, the information in thefirst MAC PDU, or the information associated with the second discoveryperiod; and determine the pseudo-random number based on the PRBS.

Example 16 is the apparatus of Example 15, wherein the control circuitryis to initialize the PRBS based on the identifier of the second UE, afield or bits of the first discovery MAC PDU, or a system frame number(SFN) or a subframe number associated with the second discovery period.

Example 17 is one or more non-transitory computer-readable media havinginstructions, stored thereon, that when executed cause a user equipment(UE) to: obtain a transmission probability for transmission of adiscovery medium access control (MAC) protocol data unit (PDU) fordevice-to-device (D2D) communications; determine a pseudo-random binarysequence (PRBS) based on an identifier of the UE, information in thediscovery MAC PDU, or information associated with a discovery period;determine a pseudo-random number based on the PRBS; and determinewhether to send the discovery MAC PDU in the discovery period based onthe pseudo-random number and the transmission probability.

Example 18 is the one or more media of Example 17, wherein theinstructions, when executed, further cause the UE to transmit thediscovery MAC PDU in the discovery period if the pseudo-random number isless than the transmission probability, and to refrain from sending thediscovery MAC PDU in the discovery period if the pseudo-random number isgreater than the transmission probability.

Example 19 is the one or more media of Example 17, wherein thepseudo-random number is determined by:

$p_{UE} = {\sum\limits_{i = 0}^{L - 1}{{c\left( {{L \cdot \left( {{10 \cdot {SFN}} + {firstSubFrameldx}} \right)} + i} \right)} \cdot 2^{- {({i + 1})}}}}$

where p_(UE) is the pseudo-random number, c corresponds to a PRBSgeneration function, SFN is a system frame number associated with thediscovery period, firstSubFrameIdx is an index of a first subframe ofthe discovery period, and L is a positive integer.

Example 20 is the one or more media of Example 17, wherein thepseudo-random number is determined by:

$p_{UE} = {\sum\limits_{i = 0}^{L - 1}{{c\left( {{L \cdot \left( {\left( {\sum\limits_{j = 0}^{M - 1}{{{DiscoveryMsg}(j)} \cdot 2^{j}}} \right){mod}\mspace{14mu} K} \right)} + i} \right)}*2^{- {({i + 1})}}}}$

where p_(UE) is the pseudo-random number, c corresponds to a PRBSgeneration function, K is a pre-defined constant, DiscoveryMsg(j), j=0,1, . . . , M−1 corresponds to a length-M set of indices of a codedversion of the first discovery MAC PDU or bits of an uncoded version ofthe first discovery MAC PDU, and L is a positive integer.

Example 21 is the one or more media of any one of Examples 17 to 20,wherein the instructions, when executed, further cause the UE toinitialize the PRBS with an initial sequence c_(init) according to:

$c_{init} = {{\sum\limits_{i = 0}^{22}{{{ProSeAppCode}(i)} \cdot 2^{i}}} + {\sum\limits_{i = 23}^{30}{{{ProSeFuncID}\left( {i - 23} \right)} \cdot 2^{i}}}}$

where ProSeAppCode(i), i=0, 1, . . . , 22, corresponds to 23 bits of aProSe Application Code or a ProSe Application ID of the discovery MACPDU, and ProSeFuncID(i−23), i=23, 24, . . . , 30, corresponds to 8 bitsof a ProSe Function ID of the discovery MAC PDU.

Example 22 is the one or more media of any one of Examples 17 to 20,wherein the instructions, when executed, further cause the UE toinitialize the PRBS with an initial sequence c_(init) according to:

$c_{init} = {\sum\limits_{i = 0}^{30}{{{DiscoveryMsg}(i)} \cdot 2^{i}}}$

where DiscoveryMsg(i), i=0, 1, . . . , 30, corresponds to 31 bits of thediscovery MAC PDU.

Example 23 is the one or more media of Example 17, wherein thepseudo-random number is determined based on a field or bits of thediscovery MAC PDU.

Example 24 is the one or more media of Example 17, wherein thepseudo-random number is determined based on a system frame number (SFN)or a subframe number associated with the discovery period.

Example 25 is an apparatus to be employed by a user equipment (UE), theapparatus comprising: means to obtain a transmission probability fortransmission of a discovery medium access control (MAC) protocol dataunit (PDU) for device-to-device (D2D) communications; means to determinea pseudo-random binary sequence (PRBS) based on an identifier of the UE,information in the discovery MAC PDU, or information associated with adiscovery period; means to determine a pseudo-random number based on thePRBS; and means to determine whether to transmit the discovery MAC PDUin the discovery period based on the pseudo-random number and thetransmission probability.

Example 26 is the apparatus of Example 25, further comprising means totransmit the discovery MAC PDU in the discovery period if thepseudo-random number is less than the transmission probability, and torefrain from sending the discovery MAC PDU in the discovery period ifthe pseudo-random number is greater than the transmission probability.

Example 27 is the apparatus of Example 25, wherein the pseudo-randomnumber is determined by:

$p_{UE} = {\sum\limits_{i = 0}^{L - 1}{{c\left( {{L \cdot \left( {{10 \cdot {SFN}} + {firstSubFrameldx}} \right)} + i} \right)} \cdot 2^{- {({i + 1})}}}}$

where p_(UE) is the pseudo-random number, c corresponds to apseudo-random binary sequence (PRBS) generation function, SFN is asystem frame number associated with the discovery period,firstSubFrameIdx is an index of a first subframe of the discoveryperiod, and L is a positive integer.

Example 28 is the apparatus of Example 25, wherein the pseudo-randomnumber is determined by:

$p_{UE} = {\sum\limits_{i = 0}^{L - 1}{{c\left( {{L \cdot \left( {\left( {\sum\limits_{j = 0}^{M - 1}{{{DiscoveryMsg}(j)} \cdot 2^{j}}} \right){mod}\mspace{14mu} K} \right)} + i} \right)}*2^{- {({i + 1})}}}}$

where p_(UE) is the pseudo-random number, c corresponds to apseudo-random binary sequence (PRBS) generation function, K is apre-defined constant, DiscoveryMsg(j), j=0, 1, . . . , M−1 correspondsto a length-M set of indices of a coded version of the discovery MAC PDUor bits of an uncoded version of the discovery MAC PDU, and L is apositive integer.

Example 29 is the apparatus of any one of Examples 25 to 28, furthercomprising means to initialize the PRBS with an initial sequencec_(init) according to:

$c_{init} = {{\sum\limits_{i = 0}^{22}{{{ProSeAppCode}(i)} \cdot 2^{i}}} + {\sum\limits_{i = 23}^{30}{{{ProSeFuncID}\left( {i - 23} \right)} \cdot 2^{i}}}}$

where ProSeAppCode(i), i=0, 1, . . . , 22, corresponds to 23 bits of aProSe Application Code or a ProSe Application ID of the discovery MACPDU, and ProSeFuncID(i−23), i=23, 24, . . . , 30, corresponds to 8 bitsof a ProSe Function ID of the discovery MAC PDU.

Example 30 is the apparatus of any one of Examples 25 to 28, furthercomprising means to initialize the PRBS with an initial sequencec_(init) according to:

$c_{init} = {\sum\limits_{i = 0}^{30}{{{DiscoveryMsg}(i)} \cdot 2^{i}}}$

where DiscoveryMsg(i), i=0, 1, . . . , 30, corresponds to 31 bits of thediscovery MAC PDU.

Example 31 is the apparatus of Example 25, wherein the pseudo-randomnumber is determined based on a field or bits of the discovery MAC PDU.

Example 32 is the apparatus of Example 25, wherein the pseudo-randomnumber is determined based on a system frame number (SFN) or a subframenumber associated with the discovery period.

Some portions of the preceding detailed description have been presentedin terms of algorithms and symbolic representations of operations ondata bits within a computer memory. These algorithmic descriptions andrepresentations are the ways used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the arts. An algorithm is here, and generally,conceived to be a self-consistent sequence of operations leading to adesired result. The operations are those requiring physicalmanipulations of physical quantities.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the above discussion, itis appreciated that throughout the description, discussions utilizingterms such as those set forth in the claims below refer to the actionand processes of a computer system, or similar electronic computingdevice, that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission, or display devices.

Embodiments of the invention also relate to an apparatus for performingthe operations herein. Such a computer program is stored in anon-transitory computer-readable medium. A machine-readable mediumincludes any mechanism for storing information in a form readable by amachine (e.g., a computer). For example, a machine-readable (e.g.,computer-readable) medium includes a machine- (e.g., a computer-)readable storage medium (e.g., read only memory (ROM), random accessmemory (RAM), magnetic disk storage media, optical storage media, flashmemory devices).

The processes or methods depicted in the preceding figures can beperformed by processing logic that comprises hardware (e.g., circuitry,dedicated logic, etc.), software (e.g., embodied on a non-transitorycomputer-readable medium), or a combination of both. Although theprocesses or methods are described above in terms of some sequentialoperations, it should be appreciated that some of the operationsdescribed can be performed in a different order. Moreover, someoperations can be performed in parallel rather than sequentially.

Embodiments of the present invention are not described with reference toany particular programming language. It will be appreciated that avariety of programming languages can be used to implement the teachingsof embodiments of the invention as described herein. In the foregoingSpecification, embodiments of the invention have been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications can be made thereto without departing fromthe broader spirit and scope of the invention as set forth in thefollowing claims. The Specification and drawings are, accordingly, to beregarded in an illustrative sense rather than a restrictive sense.

1. An apparatus to be employed by a user equipment (UE), the apparatuscomprising: communication circuitry to interface with a radiotransceiver to wirelessly communicate with an evolved Node B (eNB); andcontrol circuitry coupled to the communication circuitry, the controlcircuitry to: obtain, via the communication circuitry, a transmissionprobability from the eNB for transmission of a discovery medium accesscontrol (MAC) protocol data unit (PDU) for device-to-device (D2D)communications; determine a pseudo-random number based on an identifierof the UE, information in the discovery MAC PDU, or informationassociated with a discovery period; and compare the pseudo-random numberwith the transmission probability to determine whether to send thediscovery MAC PDU in the discovery period.
 2. The apparatus of claim 1,wherein the control circuitry is to: determine a pseudo-random binarysequence (PRBS) based on the identifier of the UE, the information inthe MAC PDU, or the information associated with the discovery period;and determine the pseudo-random number based on the PRBS.
 3. Theapparatus of claim 2, wherein the control circuitry is to initialize thePRBS based on the identifier of the UE, a field or bits of the discoveryMAC PDU, or a system frame number (SFN) or a subframe number associatedwith the discovery period.
 4. The apparatus of claim 1, wherein thecontrol circuitry is to determine the pseudo-random number based on theidentifier of the UE, wherein the identifier is a D2D UE identity thatis a function of an international mobile subscriber identity (IMSI) ofthe UE, a D2D UE identity that is a function of a system architectureevolution temporary mobile subscriber identity (S-TMSI) of the UE, or aLayer 2 identifier (ID) of the UE.
 5. The apparatus of claim 1, whereinthe control circuitry is to determine the pseudo-random number based ona field or bits of the discovery MAC PDU.
 6. The apparatus of claim 5,wherein the control circuitry is to determine the pseudo-random numberbased on a ProSe application code and a ProSe Function ID of thediscovery MAC PDU.
 7. The apparatus of claim 1, wherein the controlcircuitry is to determine the pseudo-random number based on a systemframe number (SFN) or a subframe number associated with the discoveryperiod.
 8. The apparatus of claim 1, wherein the discovery period is afirst discovery period and the discovery MAC PDU is a first discoveryMAC PDU, and wherein the control circuitry is further to adjust thetransmission probability for determination of whether to transmit asecond discovery MAC PDU in a second discovery period based on whetherthe first discovery MAC PDU was transmitted in the first discoveryperiod.
 9. The apparatus of claim 1, wherein the discovery period is afirst discovery period, and wherein the control circuitry is to comparethe pseudo-random number with the transmission probability to determinewhether to transmit the discovery MAC PDU in a set of discovery periodsthat includes the first discovery period.
 10. An apparatus to beemployed by a first user equipment (UE), the apparatus comprising:communication circuitry to interface with a radio transceiver towirelessly communicate with a second UE via device-to-device (D2D)communication; control circuitry coupled to the communication circuitry,the control circuitry to: receive, via the communication circuitry, afirst discovery medium access control (MAC) protocol data unit (PDU)from the second UE in a first discovery period; determine whether thesecond UE is to transmit a second discovery MAC PDU in a seconddiscovery period based on a transmission probability associated with thesecond UE; and monitor for the second MAC PDU in the second discoveryperiod based on the determination.
 11. The apparatus of claim 10,wherein the control circuitry is to: determine a pseudo-random binarynumber based on an identifier of the second UE, information in the firstdiscovery MAC PDU, or information associated with the second discoveryperiod; and compare the pseudo-random number with the transmissionprobability to determine whether the second UE is to transmit the seconddiscovery MAC PDU in the second discovery period.
 12. The apparatus ofclaim 11, wherein the control circuitry is to determine thepseudo-random number based on a field or bits of the discovery MAC PDU.13. The apparatus of claim 12, wherein the control circuitry is todetermine the pseudo-random number based on a ProSe application code anda ProSe Function ID of the discovery MAC PDU.
 14. The apparatus of claim11, wherein the control circuitry is to determine the pseudo-randomnumber based on a system frame number (SFN) or a subframe numberassociated with the discovery period.
 15. The apparatus of claim 11,wherein the control circuitry is to: determine a pseudo-random binarysequence (PRBS) based on the identifier of the second UE, theinformation in the first MAC PDU, or the information associated with thesecond discovery period; and determine the pseudo-random number based onthe PRBS.
 16. The apparatus of claim 15, wherein the control circuitryis to initialize the PRBS based on the identifier of the second UE, afield or bits of the first discovery MAC PDU, or a system frame number(SFN) or a subframe number associated with the second discovery period.17. One or more non-transitory computer-readable media havinginstructions, stored thereon, that when executed cause a user equipment(UE) to: obtain a transmission probability for transmission of adiscovery medium access control (MAC) protocol data unit (PDU) fordevice-to-device (D2D) communications; determine a pseudo-random binarysequence (PRBS) based on an identifier of the UE, information in thediscovery MAC PDU, or information associated with a discovery period;determine a pseudo-random number based on the PRBS; and determinewhether to send the discovery MAC PDU in the discovery period based onthe pseudo-random number and the transmission probability.
 18. The oneor more media of claim 17, wherein the instructions, when executed,further cause the UE to transmit the discovery MAC PDU in the discoveryperiod if the pseudo-random number is less than the transmissionprobability, and to refrain from sending the discovery MAC PDU in thediscovery period if the pseudo-random number is greater than thetransmission probability.
 19. The one or more media of claim 17, whereinthe pseudo-random number is determined by:$p_{UE} = {\sum\limits_{i = 0}^{L - 1}{{c\left( {{L \cdot \left( {{10 \cdot {SFN}} + {firstSubFrameldx}} \right)} + i} \right)} \cdot 2^{- {({i + 1})}}}}$where p_(UE) is the pseudo-random number, c corresponds to a PRBSgeneration function, SFN is a system frame number associated with thediscovery period, firstSubFrameIdx is an index of a first subframe ofthe discovery period, and Lisa positive integer.
 20. The one or moremedia of claim 17, wherein the pseudo-random number is determined by:$p_{UE} = {\sum\limits_{i = 0}^{L - 1}{{c\left( {{L \cdot \left( {\left( {\sum\limits_{j = 0}^{M - 1}{{{DiscoveryMsg}(j)} \cdot 2^{j}}} \right){mod}\mspace{14mu} K} \right)} + i} \right)}*2^{- {({i + 1})}}}}$where p_(UE) is the pseudo-random number, c corresponds to a PRBSgeneration function, K is a pre-defined constant, DiscoveryMsg(j), j=0,1, . . . , M−1 corresponds to a length-M set of indices of a codedversion of the discovery MAC PDU or bits of an uncoded version of thediscovery MAC PDU, and L is a positive integer.
 21. The one or moremedia of claim 17, wherein the instructions, when executed, furthercause the UE to initialize the PRBS with an initial sequence c_(init)according to:c _(init)=Σ_(t=0) ²²ProSeAppCode(i)·2^(t)+Σ_(t=23)³⁰ProSeFuncID(i−23)·2^(t) where ProSeAppCode(i), i=0, 1, . . . , 22,corresponds to 23 bits of a ProSe Application Code or a ProSeApplication ID of the discovery MAC PDU, and ProSeFuncID(i−23), i=23,24, . . . , 30, corresponds to 8 bits of a ProSe Function ID of thediscovery MAC PDU.
 22. The one or more media of claim 17, wherein theinstructions, when executed, further cause the UE to initialize the PRBSwith an initial sequence c_(init) according to:$c_{init} = {\sum\limits_{i = 0}^{30}{{{DiscoveryMsg}(i)} \cdot 2^{i}}}$where DiscoveryMsg(i), i=0, 1, . . . , 30, corresponds to 31 bits of thediscovery MAC PDU.
 23. The one or more media of claim 17, wherein thepseudo-random number is determined based on a field or bits of thediscovery MAC PDU.
 24. The one or more media of claim 17, wherein thepseudo-random number is determined based on a system frame number (SFN)or a subframe number associated with the discovery period.